Many systems use filters to selectively attenuate certain signal frequencies. Band stop filters greatly reduce signal strength within a particular band of frequencies, but otherwise permit the signal to pass through the filter without attenuation. In some cases, a filter may need to have two stop bands instead of one, selectively removing these dual bands without impacting other frequencies.
Band stop filters are also known as notch filters. Other names for such filters include band limit, T-notch, band-elimination, and band-reject. Regardless of the assigned name, all of these filters block transmission of a relatively narrow band of frequencies, where the highest blocked frequency is usually no more than one hundred times the lowest blocked frequency.
Existing techniques can couple band stop filters together, but such techniques have certain drawbacks. For example, a cross-slot iris may couple two resonating cavities, transferring a magnetic field from a first cavity to a second cavity. In conventional systems, such magnetic field transfer may involve an elongated string of cavities, where the first cavity is aligned along the same axis as the second cavity.
However, it may be difficult to provide tuning when collinear cavities are coupled by an iris. Because the iris may be disposed along the central line, it may not be possible to move the cavities once they are linked together. Moreover, it may not be easy for a user to access the iris if a large number of cavities are coupled together in a string. Such a structure may be cumbersome and difficult to store.
In addition, a known technique for combining notch filters to produce a double stop bands may produce a stretched, unwieldy structure. Cascading a first notch filter into a second notch filter, according to this conventional approach, would require an elongated transmission line, stretched out along the length of both the first notch filter and the second notch filter.
Moreover, cascading notch filters together may result in a degraded signal. While the initial notch filter would theoretically only subtract a stop band from a signal, it may also produce significant distortion and noise. This is particularly true if the initial notch filter consisted of a plurality of cavity resonators, wherein each resonator might contribute a small amount of distortion or noise. Therefore, the output of the cascaded notch filters would not produce a clean signal with two stop bands but a spectrum with significant noise and distortion.
For the foregoing reasons and for further reasons that will be apparent to those of skill in the art upon reading and understanding this specification, there is a need for an improved way of tuning a filter with two stop bands. There is also a need to produce a dual stop band characteristic on a transmission line that uses a more compact configuration. Furthermore, there is a need to produce dual stop bands without using cascaded filters.